Why Do You See Halos and Starbursts Around Lights at Night? Astigmatism vs. Glare

In an optically ideal eye (an emmetropic eye), the cornea has a perfectly spherical curvature, like the surface of a basketball. Light rays entering from all 360 degrees refract equally and converge into a single, razor-sharp focal point on the fovea centralis.

In an eye with astigmatism, the cornea is shaped like a football or the back of a spoon. One meridian (for example, the vertical axis) is steeper, while the perpendicular meridian (the horizontal axis) is flatter. Because the two meridians possess different refracting powers, incoming point-sources of light cannot converge into a single focal point. Instead, they produce two distinct focal lines separated by an interval of Sturm.

In bright daylight with small pupils, depth of field masks this optical flaw. But at night, when oncoming headlights pierce a dark roadway, the football-shaped corneal curvature stretches the point-source into long, distracting starburst streaks across your visual field.

It is clinically important to distinguish between astigmatic starbursts and circular halos:

• Astigmatic Starbursts: Look like fine radiating lines, spires, or asymmetrical light flares stretching in a specific directional angle (corresponding to your corneal cylinder axis). When you tilt your head 90 degrees, the streaks rotate along with your head.
• Diffraction Halos: Appear as hazy, circular rainbow-colored rings uniformly surrounding light sources. Halos do not rotate when you tilt your head. They are caused by light scattering through microscopic cellular edema or protein aggregates in the cornea or lens (commonly seen in cataracts, dry eye disease, or uncorrected spherical aberrations).

Correcting astigmatic night distortion requires exact ophthalmic surfacing. The prescription specifies a Cylinder (CYL) value to equalize the power difference between the two meridians, and an Axis (AX) angle (from 1° to 180°) indicating the exact orientation of the toric correction.

Even a minute alignment error of 3 to 5 degrees on the axis can cause significant residual astigmatism at night. That is why digital backside freeform surfacing with 0.01-diopter precision is critical for drivers.